But there is also a disadvantage, and that is that the 2-2-hole solution has \(A(r)\)  and \(B(r)\)  varying very rapidly for \(r\approx 2M\) . \(A(r)\)  in particular has an extremely sharp peak that grows for increasing \(M\) . This makes it numerically very demanding to study solutions with \(M\)  much greater than a few hundred in Planck units. This has been sufficient to find scaling laws for the interior region that is inside of the \(A(r)\)  peak location [1]}, [2]}, [3]}. But this leaves an important physical property of realistically large 2-2-holes quite uncertain. This is the time delay for gravitational wave echoes, that is the travel time for a wave to travel from the light-ring to the origin and back out again. The wave lingers near the peak region where the apparent speed of light drastically drops.
